Hello there,
here is a quick one on how to check for an non/empty field.
In the visibility conditions we have the “empty” and “non empty” operators.
To achieve this with a meta query and other filters, we can compare the field with the comparison operator != (not equal) or = (equal) and leave the comparison value empty. In the following example, only non-empty objects are displayed, so it is the equivalent of the “Show visibility” condition above.
array(
'key' => 'acf_field',
'compare' => '!=', // Check if the field is not equal to empty
'value' => '', // Empty value, as we're comparing to empty
),
Hope this helps 
Cheers